Handheld thermal imaging cameras, for example, including microbolometer detectors to generate infrared (IR) images, are used in a variety of applications, which include the inspection of buildings and industrial equipment. Many state-of-the-art thermal imaging cameras, or IR cameras, have a relatively large amount of built-in functionality allowing a user to select a display from among a host of display options, so that the user may maximize his ‘real time’, or on-site, comprehension of the thermal information collected by the camera.
As is known, IR cameras generally employ a lens assembly working with a corresponding infrared focal plane array (FPA) to provide an image of a view in a particular axis. The operation of such cameras is generally as follows. Infrared energy is accepted via infrared optics, including the lens assembly, and directed onto the FPA of microbolometer infrared detector elements or pixels. Each pixel responds to the heat energy received by changing its resistance value. An infrared (or thermal) image can be formed by measuring the pixels' resistances—via applying a voltage to the pixels and measuring the resulting currents or applying current to the pixels and measuring the resulting voltages. A frame of image data may, for example, be generated by scanning all the rows and columns of the FPA. A dynamic thermal image (i.e., a video representation) can be generated by repeatedly scanning the FPA to form successive frames of data. Successive frames of thermal image data are generated by repeatedly scanning the rows of the FPA; such frames are produced at a rate sufficient to generate a video representation of the thermal image data.
IR images typically show fixed pattern noise resulting from certain non-uniformities. The non-uniformities often come from physical variations between the pixels in the FPA and from stray energy detected by the FPA. Temperature changes within or surrounding an infrared camera are found to result in the individual pixels further exhibiting their unique response characteristics. In particular, the change in temperature of the camera's internal components, e.g., due to self-heating or as the result of changes to the surrounding ambient temperature, leads to the individual pixels exhibiting fixed pattern noise over extended lengths of time. Non-uniformity correction (NUC) functionality is found in most conventional infrared cameras because it leads to improved imaging capabilities. Examples of NUC methods are disclosed, for instance, in U.S. Pat. Nos. 6,690,013 and 7,417,230 and U.S. Patent Application Publication No. 2006/0279632, which are assigned to the assignee of the present invention and all of which are herein incorporated in their entirety by reference.
“Offset compensation” is one approach to NUC, which can also include ‘two-point’ (gain/offset) correction. NUC methods, can utilize a shutter or be shutterless. NUC methods using a shutter can be an inconvenience to the user as it necessitates activation of the camera shutter, thereby “freezing” the camera image for a short period of time when the shutter is closed. For example, during initial powering of an infrared camera, the internal components can be found to continue to rise in temperature for a period of time before the camera becomes thermally stable. Because of this, offset compensation is often performed at an increased frequency during such period so as to maintain good image quality from the camera. Such increased frequency of offset compensation correspondingly results in an increased frequency of shutter actuation. Consequently, there is further inconvenience for the user as the shutter is closed more often during such period. Therefore, it is desirable to keep the period between offset compensations lengthy so as to limit the general inconvenience to the user of the camera, while still maintaining good image quality.
Even with known NUC methods, whether or not a shutter is employed, non-uniformities often remain that produce various levels of fixed-pattern noise artifacts. These artifacts are due to numerous sources and typically show up as non-uniformities in an image of a uniform scene (e.g., halos, blobs, clouds, etc.). Some of these non-uniformities can be further compensated for by operating the internal shutter as described herein above. However, since the shutter is typically located between the lens and the FPA, the result is less than ideal because there can be sources of stray, i.e., non-scene, radiation that is not seen when the shutter is closed and therefore not adequately compensated for. Non-uniformities resulting from non-scene radiation, including internal stray radiation and stray radiation emitted by lens assembly and/or the lens itself, can be compensated for by using an external shutter placed over the lens outside the camera. However, external shutters usually need to be large, are costly, and are less rugged than internal shutters. Shutter temperature is needed if one wishes to perform radiometric calculations to compute the actual temperature of the scene or target. Radiometry and radiometric imaging are known and are disclosed, for instance, in U.S. Pat. No. 7,304,297, which is assigned to the assignee of the present invention and all of which is herein incorporated in its entirety by reference.
Accordingly, what is needed are an apparatus and systematic methods to address or overcome one or more of the limitations briefly described above with respect to non-uniformities resulting from fixed-pattern noise artifacts in infrared imaging systems and in such imaging systems employing radiometry.